4-bit

Some of the first microprocessors had a 4-bit word length and were developed around 1970. The TMS 1000, the world's first single-chipmicroprocessor, was a 4-bit CPU; it had a Harvard architecture, with an on-chip instruction ROM with 8-bit-wide instructions and an on-chip data RAM with 4-bit words.[1] The first commercial microprocessor was the binary coded decimal (BCD-based) Intel 4004,[2][3] developed for calculator applications in 1971; it had a 4-bit word length, but had 8-bit instructions and 12-bit addresses.

The HP Saturn processors, used in many Hewlett-Packard calculators between 1984 and 2015 (including the HP 48 series of scientific calculators) are 4-bit machines; as the Intel 4004 did, they string multiple 4-bit words together, e.g. to form a 20-bit memory address, and most of its registers are 64 bits, storing 16 4-bit digits. Its instructions were 10 bits wide.[4][5][6] Since 2003, new Saturn-based HP calculators (including the HP 49/50 series) utilize a 32-bit processor with an ARM920T core to emulate an extended Saturn processor architecture named Saturn+ at a higher speed.

The 4-bit processors were programmed in assembly language because of the extreme size constraint on programs and common programming languages (for microcontrollers, 8-bit and larger), such as C programming language, do not support 4-bit (C requires that the size of the char data type be at least 8 bits,[7] and that that all data types other than bitfields have a size that is a multiple of the character size[8][9][10]). While larger than 4-bit values can be used by combining more than one manually, the language has to support the smaller values used in the combining. If not, assembly is the only option.

The 1970s saw the emergence of 4-bit software applications for mass markets like pocket calculators.

In the 1970s and 1980s a number of research and commercial computers used bit slicing, in which the CPU's arithmetic-logic unit was built from multiple 4-bit-wide sections, each section including a chip such as an Am2901 or 74181 chip.

The Zilog Z80, although it is an 8-bit microprocessor, has a 4-bit ALU.[11][12]

Contents

Modern uses1

Details2

List of 4-bit processors3

See also4

References5

External links6

Modern uses

While 32- and 64-bit processors are more prominent in modern consumer electronics, 4-bit CPUs continue to be used (usually as part of a microcontroller) in cost-sensitive applications which require minimal computing power. For example, one popular bicycle computer specifies that it uses a "4 bit 1-chip microcomputer".[13] Other typical uses include coffee makers, infrared remote controls,[14] and security alarms.[15]

Use of 4-bit processors has declined relative to 8- or even 32-bit processors as they are hard to find cheaper in general computer supplier's stores. The simplest kinds are not available in any of them and others are "Non-stock" and more expensive[16] (a few expensive ones can be found, as of 2014, on eBay[17][18][19]).

Electronics store still carry, as of 2014, non-CPU/MCU 4-bit chips, such as counters.

As of 2015, most PC motherboards, especially laptop motherboards, use a 4-bit LPC bus (introduced in 1998) to connect the southbridge to the motherboard firmware flash ROM (UEFI or BIOS) and the Super I/O chip.[20][21]

Details

With four bits, it is possible to create 16 different values. All single digit hexadecimal numbers can be written with four bits. Binary-coded decimal is a digital encoding method for numbers using decimal notation, with each decimal digit represented by four bits.

Encyclopedia Article

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